1. Field of the Invention
The present invention relates to a method for generating digital actual velocity signals in a positioning system for the write/read heads of a magnetic disk storage, and in particular such a method in which a position signal for the precise regulation is obtained from servo information.
2. Description of the Prior Art
Positioning systems know in the art for disk storages generally have electric linear motors as the positioning drive. Such systems substantially without exception utilize a time optimal control of the track acess, meaning that the positioner is first accelerated with the full driving foce available, and is the decelerated or braked.
In such a time optimal control positioning system, the velocity must be determined according to a root function from the target distance. In conventional positioning controls, this is achieved by converting the content of a track difference register, having a current reading presenting the distance to the target track, into an analog path signal and subsequently smoothing that signa. The smoothing of the path curve is achieved by integrating the velocity signal between two successive cylinder pulses and the small path triangles that arise as a result threrof are then subtracted from the stepped path curve at the D/A converter output. The smoothed path curve is then adjusted by a root function generator to finally achieve the rated velocity curve, and is then compared at the input of asumming amplifier with the analog actual velocity signal supplied by an inductive or electronic velocity generator. The summing amplifier amplifies the difference between rated and actual velocity signals and thereby controls a power amplifier for producing an appropriate positioning current to a coil. The power amplifier is generally a linear amplifier, i.e., the coil current will be proportional to the repetitive error in the area that is not current limited.
Other approaches utilizing switching amplifiers are also known in the art. In such methods, the delay current is pulsed (two-position control) whereby the frequency of the deceleration pulses amounts only to a few kilohertz in order to avoid instability in the system. The change of the correcting variable is achieved by means of phase modulation of the deceleration pulses. An analog comparison of the repetitive error with a saw-tooth voltage is undertaken to achieve the correction. This method has the advantage in allowing for the use of a power amplifier of a simplier design however, the use of phase modulation is more complex than the linear system, so that the overall system is not substantially simplified.
Both of the above-described linear and non-linear systems utilizes inductive or electronic generators in connection with extensive analog devices. The use of such velocity generators is the chief disadvantage of such systems because inductive velocity generators tend toward resonances and, at lower velocities, are imprecise because of coupling of the positioning coil current into the generator coil. Electronic velocity generators require a large circuit outlay and must, moreover, be adapted to the respective device parameters because such generators only undertake an indirect velocity measurement. The analog devices in the control electronics are expensive, difficult to test, and require extensive auxiliary circuits for such testing and diagnosis.
One attempt to overcome these problems, i.e., to reduce the analog portion of the control electronics, is a system in which an optimum deceleration curve for the positioner is stored in digital form and is compared by a comparator with a signal which is proportional to the actual velocity of the positioner and a digital control signal for the positioner is derived from this comparison. Although the analog portion of the control electronics can be significantly reduced with devices of this type, the difficulties of the analog velocity generators usually employed in positioners for magnetic disk storages still remain.